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Backflow / vent not shutting

I just started up my sprinkler and water is shooting out of my vent. I have a water shutoff on the outside and a shutoff to my sprinker. When I turn on the water it shoots out the bottom/vent. When I slowly open the sprinkler part it still shoots out the bottom. If I run a zone the water stops coming out the bottom and flows to the sprinklers but when it stops it shoots out the bottom again. Why is the vent/backflow not shutting? Can I fix this with out replacing the whole device?

Is there a diagram that would show how the check and / or relief valve work?

Can I replace those parts? I have 2 places that I can take apart - the siphon and then a part on top. Both have springs in them. The top part does not look like I could get the plastic part and spring out of the brass fitting.